The present invention relates to methods and systems for improving cardiovascular parameters and cardiac markers of patients undergoing medical procedures involving general anesthesia. In particular, the present invention relates to methods and systems for improving cardiovascular parameters, in particular cardiac index (CI), systemic vascular resistance (SVR), and the circulating level of the cardiac protein Troponin I (cTnI) of patients during or after open heart surgery.
There are many surgical procedures that are performed under general anesthesia. One major undesired consequence of general anesthesia is hypothermia, which is a reduction of the body""s core temperature. Hypothermia causes physiologic diseration of all major body functions including that of cardiovascular and respiratory systems, nerve conduction, mental acuity, neuromuscular reaction time and metabolic rate. Countering these side effects is a major challenge both during the operation and particularly in the postoperative procedures and in intensive care units.
In open-heart surgeries, the treated patient is connected to a heart-lung machine during the open-heart phase of the surgery. However, during the period preceding this phase, it is important that the various physiological functions of the body will be in as best as possible condition prior to connection to the heart-lung machine. However, typically the cardiac index of the patient deteriorates during this period to levels below desired. That has an effect on the eventual recovery of the patient after the surgical procedure.
Generally, reduction in cardiac index is a major problem during surgical procedures performed under general anesthesia.
The present invention provides a method and system, which bring to improvement in cardiovascular parameters and a cardiac protein marker of a patient undergoing a medical procedure under general anesthesia. This is a result, in accordance with the invention, from proper control of the body""s core temperature. This control is achieved by a system comprising a heat exchanger which is in contact with the external body surface and employing a heating regime which takes into consideration the dynamic heat transfer properties of the body.
The term xe2x80x9cimprovementxe2x80x9d or xe2x80x9cimprovingxe2x80x9d when relating to cardiovascular parameters means to denote improvement in one or more of such measured parameters as compared to patients undergoing a similar procedure with the body heat not being controlled in a manner as provided for in accordance with the invention. The purpose of such improvement is in essence to maintain the cardiovascular parameters as close as possible to normal, namely similar the level of such parameters in the individual prior to undergoing the surgical procedure.
The term xe2x80x9ccardiovascular parametersxe2x80x9d is used herein to denote measurable parameters of the cardiovascular system used to determine proper (or improper) operation of the heart and the vasculature. A very important cardiovascular parameter is the xe2x80x9ccardiac outputxe2x80x9d. The cardiac output is a measure of the pumping capacity of the heart (typically measured in liters per minute), and is an important measure of proper heart function. Usually the Cardiac Output is expressed in relation to the body surface area (BSA) as the Cardiac Indexxe2x80x94L/Min/m2. Another cardiovascular parameter is the systemic vascular resistance (SVR)xe2x80x94dynes-sec-cmxe2x88x925 which represents the cardiac afterload and is usually elevated as a result of hypothermia. This eventually necessitates augmentation of cardiac work to provide perfusion to body tissues. Cardiac troponine I (cTn-I) is a regulatory protein specific for the myocardium, the level of which is measured in the serum. The cTn-I level typically rises as a result of damage to the myocardium. cTn-I levels may significantly rise during a cardio-pulmonary bypass surgery.
The term xe2x80x9ccore temperaturexe2x80x9d is used herein to denote the temperature within the body, namely that of the internal organs and tissue. Core temperature is typically measured through the rectum but may also be measured by inserting probes through a variety of other body cavities, e.g. mouth, nasal, esophageal, bladder or ear temperature probes. The term xe2x80x9csurface temperaturexe2x80x9d will be used to denote the temperature of the external body surface (which may be that of the skin or, where the skin has been damaged, e.g. in burn injury, that of the most external layers). It should be noted that the surface temperature may vary between different body portions. The surface temperature may be measured by a variety of temperature probes including, for example, an infrared sensor measuring infrared emission from a specific skin portion, probes attached to the skin such as thermal-couple devices, thermister, etc.
In accordance with a first aspect of the invention there is provided a method for improving cardiac paramaters, including, but not limited to cardiac index, systemic vascular resistance and Cardiac Troponin I in a patient undergoing a medical procedure under general anesthesia, comprising: contacting a substantial portion of the patient""s external body surface, without covering the areas where surgical procedures are performed with a heat exchanger which can transfer heat to or absorb heat from the body surface; continuously measuring parameters from the body including at least the actual body core temperature (aBCT); and in a processor, receiving data signals corresponding to the measured parameters, comparing the aBCT with a desired body core temperature (dBCT) needed in order to maintain a desired cardiovascular parameters based on the aBCT/dBCT difference, emitting a control signal to control heat transfer properties of said heat exchanger.
In accordance with another aspect of the invention there is provided a system for improving the cardiac parameters, including, but not limited to cardiac index, systemic vascular resistance and the levels of Cardiac Troponin I of patients undergoing medical treatment under general anesthesia, the system comprising:
a processor controlled heating/cooling unit coupled to a flexible heat exchanger for contacting substantial portions of a patient""s body surface and for transferring heat to or removing heat from said portions, said cooling unit having the ability to change heat transfer properties of the heat exchanger with said substantial portions:
at least one BCT-sensing device for measuring the patient""s aBCT and emitting an aBCT data signal; and
a control module for receiving data signals from measuring devices, comprising the aBCT data signal, and for emitting a control signal for controlling heat exchange properties of said heat exchanger as a function of the data signals and a dBCT needed in order to maintain a desired cardiovascular parameters.
The invention yields an improvement of cardiovascular parameters and the after load, which is a consequence of the controlled heating-induced vasodilatation or reduction in the vasoconstriction.
In accordance with one preferred embodiment of the invention the system comprises at least one sensing device for measuring a parameter indicative of the heat transfer dynamics (HTD) between the body""s surface and the body""s core. Thus, in accordance with this embodiment, such a parameter is continuously measured and a data signal corresponding to this parameter emitted from the device is fed into said processor, and this signal is factored in the control signal emitted by the processor.
The term xe2x80x9csubstantial portion of the body surfacexe2x80x9d means to denote such a portion which is sufficient to achieve a sufficient degree of heat exchange to yield the dBCT. Typically, such a substantial portion will be at least 40%, preferably at least 50% of the body""s surface.
The heat exchanger may either be provided with an internal heat or cold producing capability e.g. including a Peltier effects modules, or the heat exchanger may be linked to at least one source of cold and/or hot fluid, which fluid then circulates between such source and the heat exchanger to transfer heat/cold between the exchanger and the source. The heat exchanger is typically designed as a flexible, modular garment for wearing over portions of the patient""s body, typically covering major portions of the patient""s torso, legs, arms, shoulders and at times also portions of the patient""s skull.
A preferred heat exchanger for use in accordance with the invention is such that leaves the front, central portion of the chest exposed to permit the surgeon access to the surgical target field. Typically, the garment is designed to leave the internal thigh surfaces exposed to permit removal of arteries which are needed in the case of bypass surgery. The garment in accordance with this preferred embodiment, which also forms an independent aspect of the invention, is typically provided with adhesive strips or flaps to permit direct adhesion to the skin for fixing of the garment in situ. Such a garment is suitable, for example, for use in open heart surgery.
The control of the heat exchange properties of said heat exchanger may involve change of the heat transfer properties between the heat exchanger and the body surface which may be achieved, for example, by changing the heat conductance parameters between the body""s surface and the skin, e.g. by pumping or removing air into or from air pockets disposed between heat radiating/heat absorbing members within the heat exchanger and the skin; or preferably, by changing the temperature of the heat exchanger, which may either be a reduction in the extent of heating or cooling, halting the heating or cooling operation, or reversing the heating or cooling operation into cooling or heating, respectively In reversing, the heat exchanger acting first as a heat source will be switched to become a heat sink, or vice versa, thus reversing the direction of heat transfer.
The heat exchanger may, for example, comprise electric heating/cooling devices, e.g. Peltier devices and others. However, in accordance with a preferred, non-limiting, embodiment of the invention, the heat exchanger is of a kind having one or more conduits or fluid transfer space defined there for passing the temperature control fluid therethrough. The fluid, which is typically, though not exclusively a liquid, e.g. water, may be driven through the conduits or space by a pump or any other suitable device therefor. Such fluid thus circulates between the heat exchanger and a heat and/or cold source. The heat exchanger is typically flexible to allow it intimate contact with a body surface for efficient heat transfer therewith.
In addition to the above noted measuring devices (the BCT sensing device, the device for measuring a parameter indicative of said HTD), the system may further comprise one or more devices for measuring temperature of the circulating fluid and for emitting data signal relating thereto to the controller. At times, where the system comprises two or more such devices, at least one of which may serve as an inlet temperature sensing device for measuring temperature of the fluid as it enters the at least one conduit or fluid transfer space, and at least one other may serve as an outlet temperature sensing device for measuring temperature of the fluid as it exits the at least one conduit or fluid transfer space. The temperature drop (xcex94T) between the garment""s inlet and the outlet is a very good indicator of said HTD, since this information, together with information on the fluid""s flow rate, permits an accurate calculation of the heat transfer between the heat exchanger and the body, which depends on said HTD. Thus, in accordance with a preferred embodiment said xcex94T and the fluid flowrate are used as an HID-indicating parameter.
The heat exchanger of the invention is typically a garment which is worn over a portion of the patient""s body. Typically, the garment may be designed so as to cover at least about 40%, preferably at least about 50% of the body""s surface. In this way, the system of the invention effectively stabilizes a patient""s body temperature, at a desired body core temperature, within a minimal tolerance. A currently preferred embodiment of the invention is the application of the system for control of body temperature of patients during or after cardiac surgery. For this purpose the heat exchanger, typically in the form of a garment, may have a variety of openings permitting access for the performance of the required procedures, for parenteral administration of drugs or fluids or for drainage of body fluids. (e.g. excretions or blood).
As will no doubt be appreciated, a heat exchanger in the form of a flexible, modular garment may typically be designed to have various forms and sizes, to meet specifications of patients of various ages, weights, heights or gender to meet the specific requirements of the desired surgical procedure.
The sensing device for measuring a skin parameter indicative of said HTD (hereinafter referred to at times as xe2x80x9cHTD devicexe2x80x9d), may, in accordance with the one embodiment, include a device for measuring a temperature at a skin portion proximal to a skin portion on which the heat exchanger is applied. The HTD may then be determined for example, by either one or both of
(i) determining the rate of temperature change at said skin portion following heating or cooling of adjacent skin portions by the heat exchanger, or
(ii) by assessing the rate of change of temperature difference between the skin portion and the core during heating or cooling of the body.
Said HTD device may, in accordance with another embodiment of the invention, consist of the aforementioned at least two sensing devices for measuring temperature of the fluid as it enters the at least one conduit in the heat exchanger and the temperature as it exits from the at least one conduit. The controller, thus receiving at least two data signals relating to the measured temperature, then calculates said HTD based on the inlets or outlet temperature differential and on the fluid flow rate, which is either determined by the controls or measured by ant appropriate measuring device.
In accordance with other embodiments, said HTD device is adapted for measuring a parameter indicative of said HTD, which parameter may be one of a variety of skin and peripheral blood flow parameters. These may be determined by many techniques, e.g. by echo Doppler signal techniques, skin conductance, peripheral blood pressure, skin temperature, skin color, etc.
The determination of the heat transfer dynamics (HTD), and taking the heat transfer dynamics into consideration in the heat control regime of the patient is an important feature of the method and system of the invention. Specifically, when the HTD parameters point to the occurrence of vasoconstriction, any applied cooling should be temporarily halted or reduced. At times, it is advantageous also to reverse the heat transfer mode, temporarily heat in a cooling mode. This means that a cooling mode will involve occasional heat pulses timed and patterned according to said HTD.
The system may have a user interface permitting a user to enter a dBCT, namely a temperature set point of the system. The user interface may farther comprise control means allowing selective operation of the system in either an automatic mode, namely in a mode permitting both cooling and heating depending on the direction or deviation of the aBCT from the dBCT. In addition, the control means may also typically allow selecting a heat only mode or a cool only mode.
Typically, the heating will be limited so that the temperature at the surface of the heat exchangers, which is in touch with the body surface, will not exceed maximum temperature, e.g. a temperature of about 40xc2x0 C. and not to fall below a minimum temperature, e.g. about 15xc2x0 C.
In order to be effective in cooling or heating, The heat exchanger garment has to be fitted onto the patient""s skin. At times, there is a need to wear such a garment for prolonged periods of time, and this may give rise to a risk of pressure wounds. In order to circumvent this problem, in accordance with one embodiment of the invention, the heat exchanger has two or more patiently flow-controlled flow sub-systems, and these sub-systems may then be used intermittently, namely, one system being inflated with fluid and used, while the other being deflated and thus not exerting pressure on the skin; and vice versa. In accordance with another embodiment, the fluid transfer to the garment is temporarily halted for periods of several seconds to minutes in order to reduce the pressure onto the skin thus reducing currents of pressure.
In accordance with one embodiment of the invention, the system comprises an electric in-line fluid heating/cooling unit and the circulation fluid is directed to flow through said unit for heating or cooling. Heating or cooling of the fluid in such a unit may also be achieved by means of an auxiliary circulatory heat transfer fluid through the intermediary of a heat exchanger within said unit. In accordance with another embodiment, the system comprises at least one cold fluid reservoir and at least one hot fluid reservoir and comprises a fluid flow control system for selectably drawing fluid from these reservoirs. One advantage of having independent hot and cold fluid reservoirs, is that the switching between heating and cooling modes can be rapid.
In a system comprising independent hot fluid and cold fluid reservoirs, the flow control system is preferably adapted to permit return fluid to flow back into the reservoir from which it was drawn. It is preferred that during switching from a cold to hot fluid or vice versa, the original fluid will flow initially to the reservoir from which it was drawn, and only after the warm fluid has been exploited will the returned fluid be directed to the other reservoir. Otherwise, the cold reservoir may be heated or the hot reservoir cooled. This may be achieved by having a temperature sensing device measuring the temperature of the fluid flowing out of the heat exchanger and only when the device measures an abrupt temperature change, will the flow control system begin to direct the fluid to the new reservoir.
In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described by way of a non-limiting example only, with occasional reference to the annexed drawings: